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Show 74 INHERITANCE CHAP. XIV. Englishman, is an attribute of the male sex, and may almost be ranked as an incipient secondary sexual character. In various breeds of the fowl the males and females often differ o-reatly · and these differences are far from being the same with those which dis~ tinguish the two sexes in the parent-species, the Gallus banldva; and consequently have originated under domestication. In certain sub-varieties of the Game race we have the unusual case of the hens differing from each other more than the cocks. In an Indian breed of a white colour stained with soot, the hens invariably have black skins, and their l;>ones are covered by a b~ack peri_osteum, whilst the cocks are never or most rarely thus charactensed. P1geons offer a more interesting case ; for the two sexes rarely differ throughout the whole great family, and the males and females of t~e parent-form, the C.livia, are undistinguisbable; yet we have seen that WJth Pouters the male has the characteristic quality of pouting more strongly developed than the female; and in certain sub-varieties 28 the males alone are spotted or striated with black. When male and female English carrier-pjgeons are exhibited in separate pens, the difference in th~ development of the wattle over the beak and round the eyes is consplCuous. So tha~ here we hav: instances of the appearance of secondary sexual characters m the domestJCated races of a species in which such differences are naturally quite absent. On the other hand, secondary sexual charact ers which prop~ rl! ?elong to the species are sometimes quite lost, or greatly d1m1mshed, under domestication. We see this in the small size o~ the tusks in our improved breeds of the pig, in comparison with those of the wild boar. There are sub-breeds of fowls in which the males have lost the fine flowing tail-feathers and hackles ; and others in which there is no difference in colour bet.wee~ the t:vo sexes. In some cases the barred plumage, whiCh rn gallmaceous bil·ds is commonly the attribute of the hen, has been transferred to the cock, as in the cuckoo subbreeds. In other cases masculine characters have been partly transferred to the female, as with the splendid plumage of the golden-spangled Hamburgh hen, the enlarged comb of the Spanish hen, the pugnacious disposition of the Game hen, and as in the well-developed spurs which occasionally appear in the hens of various breeds. In Polish fowls both sexes are ornamented with a topknot, that of the male being formed of hackle-like feathers, and this is a new male character in the genus Gallus. On the whole, as far as I can judge, new characters are more apt 28 Boitard and C01·bie, ' Les Pigeons,' p. 173; Dr. F. Chapuis, • Le Pigeon Voyageur Belge,' 1865, p. 87. CHAP. XIV. AT CORRESPONDING PERIODS. 75 to appear in the males of our domesticated animals than in the females, and afterwards to be either exclusively or more strongly inherited by the males. Finally, in accordance with the principle of inheritance as limited by sex, the appearance of secondary sexual characters in natural species offers no especial difficulty, and their subsequent increase and modification, if of any service to the species, would follow through that form of selection which in my' Origin of Species' I have called sexual selection. Inheritance at corresponding periods of Life. This is an important subject. Since the publication of my 'Origin of Species,' I have seen no reason to doubt the truth of the explanation there given of perhaps the most remarkable of all the facts in biology, nan1ely, the difference between the embryo and the adult animal. The explanation is, that variations do not necessarily or generally occur at a very early period of embryonic growth, and that such variations _are inherited at a corresponding age. As a consequence of this the embryo, even when the parent-form undergoes a great amount of modification, is left only slightly modified; and the embryos of widely-different animals which are descended from a common progenitor remain in many important respects like each other and their common pTogenitor. vVe can thus understand why embryology should throw a flood oflight on the natural system of classification, for this ought to be as far as possible genealogical. vVhen the embryo leads an independent life, that is, becomes a larva, it has to be adapted to the surrounding conditions in its structure and instincts, independently of those of its parents; and the principle of inheritance at corresponding periods of life Tenders this possible. This principle is, indeed, in one way so obvious that it escapes attention. We possess a number of races of animals and plants, which, when compared with each other and with their parent-forms, present conspicuous differences, both in the immature and mature states. Look at the seeds of the several kinds of peas, beans, maize, which can be propagated truly, and see how they differ in size, colour, and shape, whilst the full- |
